C Rating understanding

[Trex3940]

Hi everyone,

I have been researching for hours about lipo C rating and whether higher or lower c ratings affect performance and flight time duration.

I'm so confused as to what discharge rate means? C rating in a nutshell means the maximum amount of current a battery can safely discharge under load. So a 10c 5000mah lipo means it can discharge at 10 x capacity (10x5000= 50,000 mah or 50 amps) 50,000 mah equates to 833 mah per minute meaning a 5000 mah lipo will take approx 6 minutes to discharge.

Surely, the higher the c rating, the shorter the flight time duration?? I have been researching all night, and people have said c rating does not affect flight time but can provide more power under load, and some say flight time is affected by c ratings. What is it??

Any explanation will be much appreciated!!

[trillian]

The C rating is the discharge rate that the Lipo is capable of. The actual rate of discharge in use depends on the amp draw of the power system.

So, if you take as an example two Lipos, both 5000mah. One is only 10C and the other is 20C. In use, they will both supply power for the same amount of time IF the max amp draw is no more than 50 amps. The difference is that the 20C LiPo can deliver 100 amps if needed.

That's putting it as concisely as possible.

[waveydavey]

Try this site for everything LiPo

https://sites.google.com/site/tjinguytech/home

A great site with loads of info

[mahbouni]

If you discharge a battery from 100% to zero % in 1 hour then you have discharged it at a rate of 1C. If you discharge it in 6 mins then it's 10C. For 4 mins it's 15C. In reality you don't want to discharge to zero %. If you were to discharge from 100% to 20% in 4 mins then it would be 15C x 80%. So for a 4 min flight your average discharge rate would be 12C. Of course this is just the average discharge rate. If you fly hard then the peaks could be 4 times that.

[Trex3940]

Yes I understand but I like to know why forums say that the c rating does not affect flight time? It's a function of power not time. Surely the higher the c rating the shorter the flight time??

[trillian]

Yes I understand but I like to know why forums say that the c rating does not affect flight time? It's a function of power not time. Surely the higher the c rating the shorter the flight time??

Only if your motor and ESC actually draw more power, otherwise a higher C rated pack of the same mah will provide just as long flight times. So it can rightly be said that C rating does not affect flight time in that flight time is a function of the power being drawn by the motor and the capacity of the battery, not the LiPo's potential max discharge rate.

It's a bit like a 1 litre bottle with a 1 inch opening and a 1 litre bottle with a 5 inch opening. If you pour both out at a rate not exceeding what the 1 inch opening can deliver, they will both take just as long to empty but obviously if you tip the one with the big opening more it will empty imediately.

[Mark_T]

Yes I understand but I like to know why forums say that the c rating does not affect flight time? It's a function of power not time. Surely the higher the c rating the shorter the flight time??

The critical part you may be missing is that the pack does not 'push' the power out to the motor, the motor 'pulls' the power out of the pack.

Also the C rating describes the safe limit on what can be pulled out, it does not however provide a hard limit on what the motor will try to pull ...

So if you run with a pack that has a C rating too low for what the motor wants to pull out, the pack will get damaged and you'll notice what people often describe as a 'lack of punch' as the voltage falls when you try to pull too much out of the pack too quickly.

So a higher C rated pack than the motor needs will let the motor pull what it wants without damaging the pack and without the voltage falling, but once you have a high enough C rating, a higher C beyond that makes no difference really as it is the motor determining what it tries to pull.

[mahbouni]

Yes I understand but I like to know why forums say that the c rating does not affect flight time? It's a function of power not time. Surely the higher the c rating the shorter the flight time??

Your flight time is determined by how much capacity you have in your battery and how much current you draw. The more current you draw the more capacity you use.

Flight time does not determine the C rating. It's kind of the other way round. Battery manufacturers advertise C ratings, which tell you how fast you can discharge the battery. If they say it's 12C then you should be able to discharge it from 100% to 20% in 4 mins. If you fly harder you will discharge it quicker. If you fly hard enough to discharge it in under 4 mins then you will exceed the 12C rating. A 60C battery can be discharged from 100% to 20% in 48 seconds. Most people buy batteries with much higher C ratings than what they actually use, not because they want to discharge the battery in 48 seconds but because they want the battery to hold a higher voltage during the discharge.

An important thing to note is that just because the C rating says you can discharge it in 48 seconds, doesn't mean you have to. You could fly sedately for 10 mins. Your flight time is determined by the power you actually use, not the power you can use if you want to.

[Mark_T]

Most people buy batteries with much higher C ratings than what they actually use, not because they want to discharge the battery in 48 seconds but because they want the battery to hold a higher voltage during the discharge.

... and because a lot of C ratings printed on the packs have very little relationship to the actual C rating of the pack ...

[trillian]

... and because a lot of C ratings printed on the packs have very little relationship to the actual C rating of the pack ...

People also buy them for a bit of bling factor at the club. Someone had some packs that said '135C'. I thought 'holy guacamole I don't want anything on my heli remotely capable of delivering that much power it's mad'!